A. Field of the Invention
The present invention relates generally to fuel management systems for internal combustion engines and, more particularly, to an air/fuel ratio management system for an internal combustion engine utilizing a gaseous fuel.
B. Description of the Prior Art
In order to obtain optimum engine operation, modern day internal combustion engines monitor the air/fuel ratio of the fuel charge to the engine to optimize engine performance. Careful monitoring of the air/fuel ratio of the fuel charge is necessary to obtain best fuel economy and low engine emissions. As used herein, xe2x80x9cfuel chargexe2x80x9d is the mixture of fuel and air provided to the engine for combustion.
There is also a trend to employ gaseous fuel with internal combustion engines. As used herein, a xe2x80x9cgaseous fuelxe2x80x9d means a fuel which is in the gaseous state at standard temperature and pressure. A few examples of a gaseous fuels used with internal combustion engines are: compressed natural gas, liquid natural gas, and liquid petroleum gas. Gaseous fuel internal combustion engines which carefully manage the air/fuel ratio are of particular benefit in heavy duty trucks.
In order to monitor the air/fuel ratio for a gaseous fuel internal combustion engine, typically an air/fuel ratio sensor, commonly an oxygen sensor, is exposed to combustion products in the exhaust gas stream of the engine. The amount of oxygen in the combustion products of a fuel charge is indicative of the air/fuel ratio of that fuel charge just prior to combustion.
One disadvantage of an air/fuel ratio sensor in the exhaust stream is that feedback to a control system is limited to the fuel charge which has already been expended. Consequently, the delay imposed between engine combustion and air/fuel ratio sensor detection results in less than optimum engine performancexe2x80x94especially during transient engine operating conditions.
Some existing gaseous engine systems add more sensors to provide improved control. Specifically, a gas mass flow rate sensor is added to the gaseous fuel line and an air mass flow rate sensor is added to the air intake pathway upstream of the engine manifold. Typically, an input air/fuel ratio is determined from these extra sensors which can be used to control air/fuel ratio alone, or in conjunction with the more traditional exhaust air/fuel ratio sensor. Although these systems usually provide more effective control over the engine""s air/fuel ratio, it is only at the expense of these additional sensors. Furthermore, these sensors are detrimental to the reliability of this system because they increase the number of likely failure points.
Existing systems also suffer from other limitations. For example, the poppet-type valve commonly used to regulate fuel flow in gaseous engines requires a relatively high gaseous fuel pressure to operate effectively. However, lower fuel line pressure is more cost effective for some applications.
Consequently, a need remains for a gaseous internal combustion engine that employs the more effective air/fuel ratio control obtained with air/fuel ratio sensing at the engine input, but without adding two extra sensors. Also, if it is desirable to use a low pressure gaseous fuel source with such an engine, then the problem of effectively regulating low pressure gaseous fuel. flow still needs to be solved.
The present invention provides improvements to systems and processes for controlling the air/fuel ratio of spark-ignited engines. One feature of the present invention is a control system responsive to sensed air mass flow and fuel mass flow inputs to an engine which does not require a dedicated air mass flow rate sensor. Instead the system senses air mass flow from other sensors using an established relationship, such as the speed density equation.
An internal combustion engine embodying this feature has a manifold and is configured to combust a mixture of air and gaseous fuel. The engine includes an air pathway coupled to the manifold for supplying air and a fuel line coupled to the manifold for supplying gaseous fuel. The fuel line has a controllable valve for regulating fuel flow through the fuel line and a first sensor providing a fuel signal corresponding to mass flow rate of fuel through the fuel line. A second sensor provides a speed signal corresponding to rotational speed of the engine. A third sensor provides a pressure signal corresponding to pressure within the manifold. A fourth sensor provides a temperature signal corresponding to temperature within the manifold. A controller responsive to the controllable valve and the first, second, third, and fourth sensors determines an air signal corresponding to the mass flow rate of air through the air pathway as a function of the speed, pressure, and temperature signals. The controller provides a valve control signal in accordance with the air signal and the fuel signal. The controllable valve is responsive to this valve control signal to adjust the rate of fuel flow through the fuel line.
Another feature of the present invention is a process which includes the steps of sensing engine speed, manifold pressure, manifold temperature, and fuel mass flow rate. Another step of the process is determining air mass flow rate as a function of sensed engine speed, manifold pressure, and manifold temperature. Yet another step is controlling fuel flow through the fuel line in accordance with air mass flow rate and sensed fuel mass flow rate.
Still another feature of the present invention is an engine, which employs an exhaust stream sensor and an input gas mass flow sensor, and derives input air mass flow from the engine speed and a manifold signal corresponding to one of temperature or pressure within the manifold. The air/fuel ratio control of this engine is performed in accordance with the sensed fuel mass flow, air mass flow, and air/fuel ratio from the exhaust stream.
Yet another feature of the present invention is the application of a rotary actuated butterfly valve responsive to a valve control signal to provide regulation of fuel flow through the fuel line. This feature provides for the use of a relatively low pressure fuel line. A controller may provide the actuating signal to the butterfly valve in accordance with an air/fuel ratio control system.
Thus, one primary object of the present invention is to provide an improved air/fuel ratio control system for a spark-ignited engine which responds to sensed fuel mass flow and air mass flow without resort to a dedicated air mass flow sensor.
Another object of the present invention is to incorporate an exhaust air/fuel ratio sensor into this system.
Still another object of the present invention is to provide for operation at low gaseous fuel line pressures.
Further objects and advantages of the present invention will be apparent from the accompanying drawings and description.